Formation of photoinduced space-charge field during in-bulk domain creation by femtosecond NIR laser irradiation in MgO:LN crystals

TL;DR

This study investigates how femtosecond NIR laser irradiation induces space-charge fields and domain formation in MgO:LN crystals, revealing effects like microtrack creation, refractive index modification, and potential for 3D photonic crystal fabrication.

Contribution

First report of photovoltaic field generation by tightly focused femtosecond NIR laser in MgO:LN, with insights into domain and refractive index modifications during in-bulk laser processing.

Findings

Photovoltaic field generated near the focus point during femtosecond laser irradiation.

Local refractive index modification is irreversible and due to photorefractive effect.

Microtracks and domains remain stable after annealing, while refractive index changes do not.

Abstract

We have studied the domain switching under NIR fs-laser irradiation in MgO:LN single crystals with special attention to the relative positions of the light-induced domains, microtracks and regions with modified refractive index in the vicinity of the focusing point. The optical imaging along X direction of the irradiated sample demonstrated the narrow microtracks and the lens-shaped regions ("lenses") located in the vicinity of the microtracks. The relative positions of light-induced microtracks, domains and lenses were revealed by overlapping of their images. We have found that the domain envelops the microtrack and partially intersects with the lens. The temperature stability of all light-induced objects during annealing was studied. It was shown that the local modification of the refractive index disappeared irreversibly while the microtrack and domain remain unchanged. The obtained local modification of the refractive index has been considered as a result of the photorefractive effect. The disappearance of the lens after annealing is caused by bulk screening of the photoinduced space-charge field by increased bulk conductivity. The generation of photovoltaic field by tightly focused NIR fs-laser irradiation close to the focusing point is reported for the first time to the authors' knowledge. It should be noted that in LN the photovoltaic field is codirectional with spontaneous polarization and thus cannot switch the polarization. However, it is possible in ferroelectrics with the opposite direction of the photovoltaic field and low value of threshold field. The revealed effect can be utilized for creation of 3D nonlinear photonic crystals by in-bulk domain engineering.